1. Field of the Invention
The present invention generally pertains to an oscillator which uses a PLL, in particular, it concerns a PLL oscillator used in electronic measuring devices which sweep broad frequency bands.
2. Discussion of the Background Art
In general, measurement instruments, such as network analyzers and oscillators that include phase-locked loops, are used to produce signals internally. The term xe2x80x9cphase-locked loopxe2x80x9d is abbreviated herein below as xe2x80x9cPLL,xe2x80x9d and an oscillator including a PLL is abbreviated as a xe2x80x9cPLL oscillator.xe2x80x9d)
FIG. 1A shows an example of a basic configuration of a PLL oscillator. In FIG. 1A, the PLL oscillator 100 is provided with a standard, signal oscillator 110, a phase comparator 120, a loop filter 130, which is a low-band wave filter, a voltage-controlled oscillator 140, and a divider 150. With reference to PLL oscillator 100 the signals produced by the voltage-controlled oscillator 140 are split by the divider 150. In the phase comparator 120, the phase difference between the split signal and the signal generated by the standard, signal oscillator is detected and a phase difference signal is produced corresponding to the phase difference. The phase difference signal is filtered by the loop filter 130 and input into the voltage-controlled oscillator 140. In this manner a feedback system is formed which acts in such a way that no phase difference is detected by the phase comparator 120. As a result, the voltage-controlled oscillator 140 oscillates synchronously with the standard, signal oscillator 110. Furthermore, by setting the dividing ratio of the divider 150 to a desired value, the voltage-controlled oscillator 140 can be made to oscillate at a desired frequency.
The total gain in one cycle in this feedback system is referred to as the loop gain. The loop gain of a PLL oscillator has important significance for the stability of the system and its response characteristics. For example, if the loop gain is increased, external noise has less effect on the system, and the stability of the feedback system can be increased. However, if the loop gain becomes too large, the internal noise is increased, and the stability of the feedback system is reduced. Therefore, optimization of the loop gain is an important problem in designing PLL oscillators. That is, it is desirable for the loop gain of the PLL oscillator to be constant within its sweep frequency band.
Since the sensitivities of voltage control oscillators are not constant, the loop gain has a characteristic which depends on the oscillating frequency. As the oscillating frequency of the voltage-controlled oscillator 140 becomes higher, its sensitivity decreases markedly. Therefore, the loop gain also decreases. Here, the sensitivity is the oscillation frequency differentiated by the input signal voltage in the voltage-controlled oscillator. Moreover, the splitting ratio of the divider, which is varied in order to produce oscillation of the desired frequency, undergoes greater changes the wider the sweep frequency band of the PLL oscillator becomes. Therefore, the loop gain undergoes still greater changes. An example of the characteristic of the loop gain is shown in FIG. 1B. The vertical axis in FIG. 1B is the loop gain of the PLL oscillator, and the horizontal axis is its oscillation frequency.
As mentioned above, it is desirable for the loop gain of a PLL oscillator to be constant within its sweep frequency band. In the prior art, therefore, a nonlinear circuit is inserted into the feedback loop in order to compensate for changes in the loop gain due to the sensitivity curve of the voltage-controlled oscillator and the splitting ratio. FIG. 2A shows the make-up of a PLL oscillator which compensates for the loop gain by using a non-linear circuit 260. In FIG. 2A, the PLL oscillator 200 is provided with a standard, signal oscillator 210, a phase comparator 220, a loop filter 230 that is a low-band wave filter, a voltage-controlled oscillator 240, a divider 250, and a non-linear circuit 260. FIG. 2A differs from FIG. 1A in that after the phase difference signal is filtered by the loop filter 230, it passes through the non-linear circuit 260 before it is input into the voltage-controlled oscillator 240. Its operation is the same as that of the PLL oscillator shown in FIG. 1A, and the voltage-controlled oscillator 240 oscillates synchronously with the standard, signal oscillator 210.
The non-linear circuit 260 is one in which the output voltage varies in a broken-line manner as a function of the input voltage. FIG. 2B shows the characteristic of the output signal with respect to the input signal in the non-linear circuit 260. Furthermore, the vertical and horizontal axes in FIG. 2B are the input and output signals in the non-linear circuit. The rate of change of the output signal with respect to the input signal, i.e., the differential gain, is shown in FIG. 2C. The horizontal and vertical axes in FIG. 2C are the input signal and the differential gain in the non-linear circuit 260. The non-linear circuit 260 has 2 breaking points, as shown in FIG. 2B. Therefore, the differential gain changes twice, in a step-wise manner, as shown in FIG. 2C.
The changes in the loop gain in the sweep frequency of the PLL oscillator 200 are compensated in such a way that they are contained within a constant range. FIG. 2D shows the characteristic of the compensated loop gain. The vertical axis in FIG. 2D is the loop gain of the PLL oscillator 200 and the horizontal axis is the oscillation frequency of the PLL oscillator 200. Since the differential gain varies in a step-wise manner in this non-linear circuit 260, the loop gain varies discontinuously. The non-linear circuit is formed in such a way that the steps in the differential gain in FIG. 2C become small so that the changes in the loop gain in FIG. 2D are made as smooth as possible, resulting in the nonlinear circuit being made more complex.
Even though the non-linear circuit 260 is formed in such a way that the steps in the differential gain are made small, the discontinuities in the loop gain are not eliminated. Therefore, especially when frequencies are swept, problems such as instability in the behavior of the feedback system arise. For example, when two PLL oscillators are made to sweep frequencies while a constant frequency difference is maintained, their oscillation frequencies do not actually change synchronously and the frequency difference between them is not strictly constant. Therefore, it is desirable to provide a PLL oscillator with a compensation function, such that the loop gain becomes constant without sacrificing continuity.
The present invention solves the problems in the prior art described above by compensating for changes in the loop gain such that the loop gain becomes constant, without sacrificing continuity, by providing a variable-gain amplifier in the loop of the PLL oscillator and by compensating the signal voltage input into the voltage-controlled oscillator.
A phase-locked loop oscillator, PLL, provided with a phase comparator, a wave filter, and a voltage-controlled oscillator that oscillates synchronously with a standard signal. The phase-locked loop oscillator is provided with a variable-gain amplifier the amplification rate of which can be controlled by a control device. The PLL detects signals in the loops by means of the control device and controls the variable-gain amplifier and compensates for the loop gain.
The control device controls the amplification rate of the variable-gain amplifier based on signals detected between the wave filter (e.g., loop filter) and the voltage-controlled oscillator.
The variable-gain amplifier is placed between the aforementioned phase comparator and the aforementioned wave filter.